This invention relates to instrument control circuits and, more particularly, to an electric circuit which utilizes a photoconductive sensor for detecting when a gauge indicator is in a control zone for regulating the flow of electric current to an instrument.
Many laboratory and industrial instruments are monitored by visually readable gauges which indicate when the instrument is operating within safe or optimum parameters. One such instrument is a high-speed liquid chromatograph where liquid is circulated under relatively high pressures. It is essential that the liquid be maintained within a predetermined pressure range in order to protect the machine from damage and prevent an accident from occurring. Because these instruments must be run for relatively long periods of time it is impractical for an operator personally to monitor the pressure gauge to make sure that proper operating conditions are maintained. Although gauges are available which have built-in electric, contact-type limit switches which automatically turn off the machine when the gauge indicator moves out of a predetermined control zone, these devices are not totally reliable because the contacts can corrode in the laboratory atmosphere or otherwise become ineffective and fail to turn off the instrument when a critical pressure is reached.
As an alternative to mechanical switching devices, photoconductive gauge monitoring devices have been developed which use one or more photoconductive light sensors for determining when the gauge indicator either enters or leaves a predetermined zone on a gauge. However, none is known which can simply be connected to an existing gauge without electrical or mechanical modification of the gauge or instrument and which provides fail-safe features which are essential when operating expensive equipment at high pressures or temperatures. Further, none of the known monitoring systems uses a single photoconductive sensor in conjunction with a circuit which allows the instrument to continue to operate in a cycle mode or alternatively to turn the instrument on or off at either a high or low setting and, in addition, provides a fail-safe feature such as an automatic turn off for the instrument if the lamp which is used to illuminate the photoconductor burns out or if the sensing circuit should otherwise malfunction.